When listing three or more terms/items, place a comma after each one and use ‘and’ before the last listed word. For example: “The recipe calls for milk, sugar, and vanilla extract.”
The reasoning behind this rule is to create consistency, increase readability, and decrease confusion. Consider the following: "I would like to thank my parents, Tolkien and Sanderson." In this example, the writer is meaning to thank their parents, and two authors: J.R.R. Tolkien and Brandon Sanderson. But, without the Oxford comma, it can also be read that the writer is thanking their parents who are J.R.R. Tolkien and Brandon Sanderson.